1. Field of the Invention
The present invention relates to a semipermeable membrane and a process for preparing the membrane and, more particularly, to a semipermeable membrane capable of being used for blood purification, particularly blood dialysis therapy or blood filtration dialysis therapy for treatment with kidney diseases, drug poisoning and so on.
2. Description of Related Art
Heretofore, semipermeable membranes are used in the industrial field for condensation and purification of beer, juices, milk and so on, desaltination of sea water, and sewage treatment, and in the medical field for artificial dialyzers for use with blood dialysis therapy and blood filtration dialysis therapy.
As materials for semipermeable membranes have been used cellulose, cellulose ester, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, ethylene-vinyl alcohol copolymer, polyamide and the like. Recently polysulfone resins, polyether sulfone resins, polyarylate resins and so on which have been developed as engineering plastics are utilized as a material for a semipermeable membrane in the medical field due to their favorable mechanical strength, thermal resistance and chemical resistance as well as excellent biocompatibility.
Generally, when a semipermeable membrane with an excellent permeability of waste products, water and so on is formed using the polyarylate resin or polysulfone resin only, it is required that a structure of the membrane be formed so as to allow its cross section to provide a high permeability while a high degree of denseness on membrane surfaces are prevented. It is thus desired that its cross section in the thickness direction is of a network structure, namely, of a fibrilliform structure, in which continuous micropores penetrate through the membrane in the direction of thickness thereof. In other words, a void structure or a so-called sponge-like structure having a less quantity of penetrating micropores is not preferred.
It is to be noted, however, that a semipermeable membrane formed using such a polyarylate resin or polysulfone resin as a sole ingredient may provide a membrane surface with a remarkably dense structure on account of a strength for its intermolecular aggregation and has the likelihood to provide a void structure, or a sponge-like structure, in which a polymer structure lacks in a cross-sectional direction of the membrane. Such membranes suffer from the disadvantages that a permeability of waste products in the blood, water and the like is low, their voids penetrate through their cross section up to the surface thereof, and a mechanical strength is impaired as well.
Inorder to solve these disadvantages and drawbacks, various attempts have been made so far. For example, Japanese Patent Kokai Nos. 293,502/1986, 20,085/1986, and 47,367/1987 disclose methods in which there is used as a stcok solution for forming membranes a mixture of a polysulfone resin as a semipermeable-membrane material and a solvent with an additive such as a metal salt, e.g., lithium chloride, calcium chloride or the like, or a water-soluble polymer, e.g., polyvinyl pyrrolidone, polyvinyl acetate, polyethylene glycol or the like, or a non-solvent or a swelling agent such as water, methanol, ethanol, isopropanol, ethylene glycol or the like. An addition of the additive, non-solvent, swelling agent or the like is to control a surfacial or cross-sectional structure of the membrane. The semipermeable membranes as disclosed in the above patent applications are relatively small in a water-permeable amount and excellent in albumin fractionation, however, the additive, non-solvent or swelling agent should be thoroughly removed after a formation of the semipermeable membranes. If such a substance would be remained unremoved from the membrane, it may be eluated into the blood from the membrane during blood dialysis. This affects patients seriously.
Japanese Patent Kokai Nos. 245,805/1986, 197,006/1986 and 238,834/1986 disclose other semipermeable membranes formed using a polysulfone resin. Although they are superior in mechanical properties, they are large in a water-permeable amount and in pore size so that they present problems that even useful materials in the blood are filtered off when they are used for the blood dialysis therapy.
Examples of semipermeable membranes formed from a polyarylate resin are disclosed in Japanese Patent Publication Nos. 27,964/1983 and 34,166/1983. The semipermeable membranes may be used for fractionating a substance having a small molecular weight, for example, polyethylene glycol having molecular weights ranging from approximately 6,000 to 20,000. If these membranes would be used for the blood dialysis therapy, they may remove a substance having a relatively low molecular weight, such as urea, creatinine or uric acid. However, it would be difficult to remove .beta..sub.2 -microglobulin (molecular weight: 11,800) with a high removal percentage, which is considered to be a casual substance of amyloidosis that is caused to develop at a high rate on patients who are subjected to dialysis for many years.
On the other hand, a selectively gas-permeable membrane is disclosed in Japanese Patent Kokai No. 127,602/1984, which is used in the technical field comletely different from the field in which a liquid such as the blood is filtered. The membrane may be formed using an aromatic polyester having a recurring unit as represented by following formula: ##STR1## and a polymer having a recurring unit as represented by following formula: ##STR2##
The above patent publications state that the selectively gas-permeable membrane is of a structure containing a dense layer with unsymmetrical pore sizes having an average pore size ranging from 0.001 to 0.01 .mu.m. More specifically, the membrane is of a void structure in which inner pores extend to the bottom surface in a vertical direction with respect to the membrane surface and arranged in a regular orientation. Furthermore, a sponge-like structure of wall portions partitioning the inner pores from each other is extremely dense. In other words, the selectively gas-permeable membrane is of no uniformly fibrilliform structure because the structure has unsymmetrical pore sizes and is of the void structure in which the inner pores develop in a slender shape.
It is presumed that this structure is created by steps of coagulating a stock solution which involve immersing it in a non-solvent or water or which involve partially evaporating a solution by heating after the stock solution has been casted.
The above patent publications further state that an average pore size increases as a ratio of the aromatic polyester to a polymer admixed therewith approaches 1 to 1, thus disorganizing a dense layer on the surfacial side of the membrane and enlarging a gas permeability yet reducing a selective gas permeability. It is expressly stated that this is based on the fact that a reduction in miscibility with heterogeneous polymers becomes to the maximum level so that the admixture ratio around 1 to 1 is not preferred.